Re-chargeable batteries are widely used in a lot of portable or mobile electrical and electronic devices or appliances such as, cellular or cordless telephones, remote repeaters, remote control units, remote sensors, portable lighting devices, portable radios, portable drills and many other devices. Re-chargeable batteries are generally preferred over disposable batteries nowadays because they are more environmental friendly and provide longer term cost savings. For remote applications, rechargeable batteries are probably the only practical choice.
Re-chargeable batteries require repeated charging in order to supply electrical power to the devices or appliances in which they are installed. Nowadays, portable devices usually require a plurality of batteries to operate and the batteries required are typically in the range of two to ten batteries. Hence, it is desirable that there can be provided intelligent battery chargers which can charge a plurality of re-chargeable batteries at the same time. There are two main types of battery chargers. The first type is the parallel charger in which all the batteries are subject to the same charging voltage but are charged with different charging currents. The other type is the serial charger in which the batteries being charged are connected in series and the same charging current usually passes through all the serially connected batteries.
In applications in which batteries are alternatively charged and discharged, a power supply of 3 to 12 volts is generally required while the voltage of each rechargeable battery is typically in the region of 1-2 volts. In those applications, batteries are typically connected in series for charging and discharging. For charging batteries for use in such applications, a serial battery charger must be used.
Because of the wide-spread use of rechargeable batteries, there are increasing demands for fast battery chargers which are capable of fully charging an empty battery in about an hour (the “1C” chargers) so that users do not have to wait for too long before the batteries are sufficiently charged for use. For example, for a 1,600 mAH re-chargeable battery, the 1C current rate is about 1.6A. In order to facilitate fast and efficient battery charging, battery chargers generally utilise high frequency pulsed charging current having a relatively high current rate. When a battery is being charged, it will produce oxygen on the electrode and the consumption of oxygen by the negative electrode will cause the battery to heat up. In general, charging at the current rate of 1C is preferred because this charging rate is regarded as striking a balance between reducing charging time and maintaining a healthy battery under current battery technologies. Of course, with further advance in battery technologies, batteries may be charged at even higher current ratings without over-heating. If that happens, battery chargers supplying higher charging rating than 1C will be expected to become more popular. In general, fast battery chargers, especially those for charging small voltage re-chargeable batteries of about 1.5-2V, are preferably configured so that the batteries are charged in series. This is because if the batteries are fast charged in parallel, a power supply having a very large current supply rating will be required and this may be very costly.
On the other hand, a serial connection implies that the same current must flow through each serially connected charging section. This may also create great difficulty in a lot of circumstances. For example, when a battery is removed from the charger upon completion of charging to avoid overheating or damaging or because it is already defective, charging will be disrupted until a replacement battery has been inserted into the charger. Similar problems also arise if rechargeable batteries of different capacities are charged together or good batteries are mixed with bad ones. This is because when a battery of a smaller capacity has been fully charged, there is a good chance that a battery of a larger capacity still requires charging. For simple serial chargers with no monitoring and control circuits, the batteries will be continuously charged. As a result, overheating, battery damage or even explosion may result. On the other hand, for those more sophisticated serial battery chargers with charging conditions monitoring and charge control circuits, the battery charger may shut down once any one of the batteries being charged is detected as being fully charged. This is obviously undesirable as the remaining batteries may still require further charging. Furthermore, whenever batteries are inserted or removed from a serial battery charger during the charging process, the whole charging process will be interrupted. Hence, it is desirable if there can be provided intelligent serial battery chargers which allow serial charging of re-chargeable batteries in which the charging currents supplied to the individual batteries in serial connection are largely independent of that supplied to other batteries.
For many battery chargers, it is known that, when power supply to the battery charger is turned off, there may be a reverse leakage current which flows from the battery to the charger or the peripheral circuitry. Reverse leakage current among the serially connected batteries could also cause reverse charging of individual batteries by other batteries that are connected in the series charger. This is clearly undesirable which may cause draining of the full battery capacity and may even damage the charger. Hence, it is desirable that each charging section of a serial battery charger is provided with means to prevent undesirable reverse current leakage as well as a by-passing circuitry so that the charging conditions of one individual charging section would not affect the charging conditions of the other charging sections.
Many by-passing circuits, circuit arrangements or topologies have been proposed to alleviate the adverse influence of the charging conditions in a serial charging section to other charging sections. While serial chargers having arrangements to by-pass some or all of the charging sections have been known, they are generally very complicated and do not simultaneously include means or circuits to prevent reverse leakage or discharge from the batteries.
To provide a serial battery charger which fulfils the above requirements is a difficult task because several conflicting requirements need to be met. Firstly, in order to prevent reverse current leakage or adverse current discharge from the battery, a blocking device which has a high reverse impedance must be inserted in series with the battery. Secondly, that serial block device must have a low impedance when there is a forward current which flows into the battery for battery charging. On the other hand, if the blocking device has a low forward impedance when the by-passing switch has been activated (which usually occurs when there is still power supply to the battery charging terminals), that low-impedance blocking device will compete with the by-passing switch for the supplied current and, as a result, adverse charging current will keep flowing into the batteries. In addition, that blocking device must have a high impedance when the by-passing switch has been activated, otherwise, a large and un-desirable current will flow in a current loop which is formed by the battery, the blocking device and the by-passing switch. Hence, it is highly desirable if a serial battery charger which can fulfil the above conflicting requirements can be provided. It will be even more desirable if such improved battery chargers can be realised using simple circuit blocks and components so that high reliability as well as low costs can be achieved.